Laboratory studies on the influence of the earthworm Eukerria saltensis (Beddard) (Oligochaeta : Ocnerodrilidae) on overlying water quality and rice plant establishment

Dense infestations of a peregrine oligochaete worm, Eukerria saltensis (Bed dard), have been linked to rice crop failures in southern New South Wales, Australia. The influence of E. saltensis on water quality and rice plant es tablishment was investigated in a series of laboratory experiments using a flooded Riverina clay soil. Worm densities of 20 and 40 per container (2548 and 5096 worms/ m(2), respectively) significantly increased water turbidit y after 7 days incubation. Longer incubation periods led to turbidity level s of over 500 NTU being achieved (40 worms per container, 21 days incubatio n). Water pH was significantly reduced by densities of 10, 20, and 40 worms per container after 7 days under cyclical illumination, however in continu ous darkness significant changes in pH related to worm density were only fo und after 21 days incubation. Nitrogen as NH4+ and total phosphorus increas ed significantly in the overlying water in response to increasing worm dens ities after 21 days incubation, however nitrogen as nitrate/nitrite and sol uble phosphorus did not. Algal production (measured as extracted chlorophyl l a concentration) was unaffected by the worms, reflecting the low concentr ations of available phosphorus in all treatments. Rice plants grown in cont ainers with worms produced significantly longer and heavier shoots than con trol plants. Root systems were unaffected, and there was no evidence of roo t abrasion. Evaluation of rice seed stratification in the soil profile indi cates that rice seeds can be passively transported below the soil surface b y the feeding and tunnelling activity of E. saltensis. Our results suggest that E. saltensis impedes the establishment of aerially-sown rice crops pri marily by increasing water turbidity. Plants respond to high turbidity by p artitioning more of their growth into shoot production, and consequently be come vulnerable to uprooting through wave action, particularly as the soil loses compaction because of worm activity. Maintaining the lowest possible water levels during rice crop establishment has helped farmers to minimize these effects.